1. Field of the Invention
The present invention relates to a process for removing zinc salts from nonaqueous synthesis solutions comprising zinc alkoxides or zinc amides.
2. The Prior Art
Organozinc compounds are industrially significant organometallic compounds for preparing synthetic building blocks for active pharmaceutical ingredients, scents and plant protectants.
For example, α-bromozinc esters, prepared by reaction of α-bromocarboxylic esters with zinc, find use in reactions with electrophilic substrates, for example aldehydes, ketones or imines (Reformatsky syntheses). Asymmetric additions of dialkyl- or diarylzinc compounds to aldehydes or ketones for preparing chiral alcohols as synthetic intermediates are of increasing industrial interest.
When organozinc compounds are used in industrial synthetic processes, the workup results in inorganic salts containing zinc ions or zinc salts having additional organic constituents which are typically dissolved, suspended or emulsified in an aqueous medium. As heavy metal ions, especially as a constituent of aqueous or aqueous-organic wastewater, zinc ions are generally regarded as environmentally polluting, because they cannot be destroyed, merely chemically changed, for example by conversion to soluble or insoluble zinc compounds or by reduction to the elemental state. Aqueous solutions, suspensions or emulsions comprising zinc ions therefore have to be freed of zinc ions to a legally stipulated residual content before purification, incineration, landfill disposal or disposal as appropriate wastewater.
To reduce the zinc ion content in aqueous solutions, suspensions or emulsions, in some cases comprising organic constituents, as obtained, for example, after workup of synthesis solutions or reaction mixtures in industrial preparative processes using organozinc compounds, various procedures of precipitating sparingly soluble zinc salts and removing the sparingly soluble salts from the aqueous phase are known.
U.S. Pat. No. 4,678,584 describes a method for removing zinc ions from wastewaters by precipitating as sparingly soluble sulfides by treating the wastewaters with trithiocarbonate.
U.S. Pat. No. 5,762,807 discloses a process for precipitating complexed and noncomplexed heavy metals, for example zinc, from aqueous media by treating with an aqueous mixture of polysulfide and polysulfane compounds.
EP 0526997 discloses the removal of dissolved chelated zinc ions from aqueous solutions in a multistage process by precipitating sparingly soluble zinc salts.
However, the prior art processes have a series of disadvantages. In the existing processes, the heavy metal salts are in principle precipitated from aqueous solutions, as occur after hydrolytic workup of the synthesis solutions or reaction mixtures when using organozinc compounds. This means that a medium comprising aqueous zinc ions is first generated, from which the zinc salts have to be removed again in a subsequent step by precipitation, and that the wastewaters obtained after the removal of the zinc salts have to be purified. In addition, this procedure associated with considerable cost and inconvenience is very uneconomic. For economic and ecological reasons, a significantly more desirable process would generate no aqueous phase at all and no wastewater containing zinc ions.
In addition, after removal of the precipitated heavy metal salts, wastewaters are typically obtained which contain organic constituents and organic, water-miscible solvents to a not inconsiderable extent, for example tetrahydrofuran which serves as a solvent in organozinc syntheses. For economic reasons and to reduce the amounts of waste, especially on application on the industrial scale, recovery of the solvent used from the aqueous phase, for example by extraction or distillation, is necessary, although this is again associated with a considerable financial and technical burden.
There is also the possibility that the reaction products prepared or their solutions, which may complex with the zinc ions via functional hydroxyl, carboxyl, thio or amino groups, may decompose or be contaminated by undesired by-products on treatment with the precipitating agent (e.g. sulfide, polysulfide, polysulfane, trithiocarbonate) under the conditions which are necessary for precipitating and removing the zinc salts. This is especially as a result of thermal stress or long reaction times, and thus the quality of the products prepared is reduced and the economic viability of the overall preparative process is impaired.
The complexing of the zinc salts via functional groups, e.g. oxy, amine, amide or thio groups, of the products prepared by organozinc synthesis leads to some or all of the zinc as an impurity to be removed remaining in solution.
In addition, the precipitation of the sparingly soluble zinc salts according to U.S. Pat. No. 5,762,807 and EP 0526997 entails a series of further process steps. For example these further steps include the setting of a predetermined pH by adding acid or base, the addition of further reagents or filtering assistants or the thermal treatment of the precipitate, in order to achieve better filterability. The entire precipitation process known from the prior art makes special plants necessary, which leads to considerable time demands and costs, and impairs the economic viability of the existing precipitation processes, especially when used on the industrial scale.
The zinc salts precipitated from aqueous medium, e.g. zinc hydroxide, zinc carbonate or zinc sulfide compounds or mixtures thereof are often difficult to filter and, after removal by filtration or centrifugation, contain considerable amounts of water. Since the precipitated zinc salts have to be disposed of in landfills or fed to zinc recycling, the economic viability of the entire process, especially when carried out on the industrial scale, is compromised by the considerable economic demands of transporting voluminous zinc salts having a high water content or drying or otherwise treating the zinc salts. None of the prior art processes makes it possible to directly remove zinc salts from nonaqueous synthesis or reaction solutions comprising zinc compounds without preceding aqueous workup, or under nonaqueous precipitation conditions or using nonaqueous precipitation media.